SPEEDOMETER IS OFF ABOUT 10-15%

[Neuron]

My newly purchased 85 had the old 3 speed auto replaced with a 700R4 and near as I can tell it is highly optimistic. I will get someone soon to follow me on the freeway to confirm the exact amount. Is there a gear on the end of the speedo cable that is easily changed or do you have to get into the transmission to replace the gear?

 

[bill]

the speedo is reading the transmission output shaft revs. I would assume its lower than it should be. Most of the time, when you go from a 3speed to a 4speed, the speedo reads lower mph than it should. Thats due to the difference in the rear axle ratio combined with the 4th gear of the trans. Either changing the rear gear or changing the speedo gear will correct this. Ive got the same issue with my 2004R, it reads 60mph when im actually going about 70. But, when I change my rear gear from the 2:41 to 1 to the 3:23 to 1, it should be a lot closer to reality.

 

[tc1959]

http://www.transmissioncenter.net/speed ... ____va.htm
Check this out they have every thing you need to fix this problem.

 

[pontiacgp]

I'm planning on one of these so it won't matter on gears or tire sizes..and it does a bit more than a speedo


http://www.speedhut.com/gauge/GRM4-GPS- ... m-h-Metric

 

[Neuron]

the speedo is reading the transmission output shaft revs. I would assume its lower than it should be. Most of the time, when you go from a 3speed to a 4speed, the speedo reads lower mph than it should. Thats due to the difference in the rear axle ratio combined with the 4th gear of the trans. Either changing the rear gear or changing the speedo gear will correct this. Ive got the same issue with my 2004R, it reads 60mph when i'm actually going about 70. But, when I change my rear gear from the 2:56 to 1 to the 3:08 to 1, it should be a lot closer to reality.

That is weird because mine is reading to high. On the freeway and put the speedo on 85MPH and I am staying up with traffic at around 75MPH I would guess(speed limit is 70MOPH). Now that I think about it it should be reading lower with OD trans.I have the receipt for the trans work and it does say 700R4 and I looked under neath the car and the drive shaft has been modified.I will have to go under the car and look at the trans. Is there something I should look for to ID it as a 700R4?

 

[Neuron]

I'm planning on one of these so it won't matter on gears or tire sizes..and it does a bit more than a speedo


http://www.speedhut.com/gauge/GRM4-GPS- ... m-h-Metric

That is pretty cool,did you see a accuracy measurement on it anywhere?

 

[pontiacgp]

I haven't except for "5 Htz update for fast needle response and accuracy" but many of us rely on our Garmin's or equivalent for accurate speeds to test our speedos and I would assume these are as good if not better. I have not found any complaints online about their speedos

 

[bill]

the speedo is reading the transmission output shaft revs. I would assume its lower than it should be. Most of the time, when you go from a 3speed to a 4speed, the speedo reads lower mph than it should. Thats due to the difference in the rear axle ratio combined with the 4th gear of the trans. Either changing the rear gear or changing the speedo gear will correct this. Ive got the same issue with my 2004R, it reads 60mph when i'm actually going about 70. But, when I change my rear gear from the 2:56 to 1 to the 3:08 to 1, it should be a lot closer to reality.

That is weird because mine is reading to high. On the freeway and put the speedo on 85MPH and I am staying up with traffic at around 75MPH I would guess(speed limit is 70MOPH). Now that I think about it it should be reading lower with OD trans.I have the receipt for the trans work and it does say 700R4 and I looked under neath the car and the drive shaft has been modified.I will have to go under the car and look at the trans. Is there something I should look for to ID it as a 700R4?

do you have a tachometer hooked up? if so, what rpms are you turning at highway speeds?

 

[Neuron]

the speedo is reading the transmission output shaft revs. I would assume its lower than it should be. Most of the time, when you go from a 3speed to a 4speed, the speedo reads lower mph than it should. Thats due to the difference in the rear axle ratio combined with the 4th gear of the trans. Either changing the rear gear or changing the speedo gear will correct this. Ive got the same issue with my 2004R, it reads 60mph when i'm actually going about 70. But, when I change my rear gear from the 2:56 to 1 to the 3:08 to 1, it should be a lot closer to reality.

That is weird because mine is reading to high. On the freeway and put the speedo on 85MPH and I am staying up with traffic at around 75MPH I would guess(speed limit is 70MOPH). Now that I think about it it should be reading lower with OD trans.I have the receipt for the trans work and it does say 700R4 and I looked under neath the car and the drive shaft has been modified.I will have to go under the car and look at the trans. Is there something I should look for to ID it as a 700R4?

do you have a tachometer hooked up? if so, what rpms are you turning at highway speeds?

Around 2000-2200 RPM on the freeway, I was not able to get somebody to go with me on the freeway yet to get a better handle on the true speed of the car. I climbed underneath and found MD8 on the transmission and the pan matches the 700R4/4L60 diagram so I am sure it is a 700R4.The tires are 215 70 R14. I would venture a guess the rear end ratio is stock judging b y the grease on the pumpkin(car has 112000 miles on it). The new quadrant was not installed for the upgrade to the 4 speed, but it matches with old quadrant but I would imagine that the D now means OD and 2 means non overdrive drive. I wonder if a 4 speed quadrant from a donor would just bolt in with no modification.